Processing method of glass base material for optical fiber

ABSTRACT

Provided is a method of processing a glass base material for optical fiber in which the glass base material for optical fiber is elongated to reduce a diameter thereof until reaching a final elongation diameter and form a completed base material. The method includes measuring an outer diameter distribution that includes an outer diameter of the glass base material for optical fiber; setting an effective region; calculating a target elongation diameter that is larger than the final elongation diameter and less than an average diameter of the effective region, and elongating the glass base material for optical fiber until reaching the target elongation diameter; and after reaching the target elongation diameter, further elongating the glass base material for optical fiber until reaching the final elongation diameter.

BACKGROUND

The contents of the following Japanese patent application areincorporated herein by reference:

NO. 2014-087951 filed on Apr. 22, 2014.

1. Technical Field

The present invention relates to a processing method of a glass basematerial for optical fiber.

2. Related Art

A glass base material for optical fiber is heated, softened, andelongated to form a completed base material that has an average outerdiameter, an outer diameter fluctuation, and a length suitable to adrawing device, and then this base material is drawn by the drawingdevice to form optical fiber.

If there is a large amount of outer diameter fluctuation across theentire length direction of the glass base material prior to theprocessing or if there is outer diameter fluctuation within a relativelyshort region, then it is possible that the diameter of the thick portionof the glass base material is not sufficiently reduced, which results ina large amount of outer diameter fluctuation in the completed basematerial after the processing.

SUMMARY

According to a first aspect of the present invention, provided is amethod of processing a glass base material for optical fiber in whichthe glass base material for optical fiber is elongated to reduce adiameter thereof until reaching a final elongation diameter and form acompleted base material. The method includes, before elongating theglass base material for optical fiber, measuring an outer diameterdistribution that includes an outer diameter of the glass base materialfor optical fiber at a plurality of measurement points in a longitudinaldirection of the glass base material for optical fiber; setting aneffective region that is continuous in the longitudinal direction in theglass base material for optical fiber, based on the measured outerdiameter; calculating a target elongation diameter that is larger thanthe final elongation diameter and less than an average diameter of theeffective region of the glass base material for optical fiber that iscalculated based on the measured outer diameter, and elongating theglass base material for optical fiber until reaching the targetelongation diameter; and after reaching the target elongation diameter,further elongating the glass base material for optical fiber untilreaching the final elongation diameter.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a glass lathe 1.

FIG. 2 is a block diagram of the processing procedure of a glass basematerial for optical fiber 3.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some embodiments of the present invention will bedescribed. The embodiments do not limit the invention according to theclaims, and all the combinations of the features described in theembodiments are not necessarily essential to means provided by aspectsof the invention.

FIG. 1 is a schematic view of a glass lathe 1 that can be used whenprocessing a glass base material for optical fiber 3 to form a completedbase material. The glass lathe 1 includes a diameter measurement device4 and a heat source 5.

The glass base material for optical fiber 3 that is to undergo theprocessing may be manufactured using VAD, OVD, or the like.Specifically, a porous glass base material is formed by supplying aburner with glass raw material such as silicon tetrachloride along withoxygen gas and hydrogen gas to cause a flame hydrolysis reaction anddepositing the generated glass microparticles on a starting substrate,and then performing dehydration and transparent vitrification on theresulting material to form the glass base material for optical fiber 3.The glass base material for optical fiber 3 obtained in this manner hasan overall shape that is substantially cylindrical.

When performing processing with the glass lathe 1, glass support rods(dummy rods) 2 are fused to both of the longitudinal ends of the glassbase material for optical fiber 3. Chucks of the glass lathe 1 grip theglass base material for optical fiber 3 at the dummy rod 2 portions. Inthis way, the surface of the glass base material for optical fiber 3itself is prevented from being damaged by the chucks.

Furthermore, when heating regions of the glass base material for opticalfiber 3 near the ends thereof, the heat damage experienced by the chucksdue to the heat generated by the heat source 5 can be reduced.Cylindrical glass rods with a small amount of outer diameter fluctuationare preferably used as the dummy rods 2. In this way, the axial skewoccurring when the glass base material for optical fiber 3 is rotated bythe glass lathe 1 can be reduced.

With the glass lathe 1, the glass base material for optical fiber 3 isprocessed to form the completed base material having an average outerdiameter, an outer diameter fluctuation value, and a length suitable toa drawing device that is to be used. This completed base material isthen drawn by the drawing device to form optical fiber. The outerdiameter fluctuation value can be calculated as the difference betweenthe maximum outer diameter and the minimum outer diameter of the glassbase material for optical fiber 3 as measured across the effectiveregion thereof.

When processing the glass base material for optical fiber 3 to form thecompleted base material, the chucks of the glass lathe 1 are rotatedand, while rotating the glass base material for optical fiber 3, theheat source 5 is moved relative to the glass base material for opticalfiber 3 in the longitudinal direction of the glass base material foroptical fiber 3. Furthermore, while heating the glass base material foroptical fiber 3, the space between the left and right chucks of theglass lathe 1 is widened, thereby reducing the outer diameter of theglass base material for optical fiber 3 while elongating the glass basematerial for optical fiber 3 in the longitudinal direction.

Before being machined to form the completed base material, the glassbase material has an almost cylindrical shape at the beginning, butstill has an outer diameter that fluctuates along the longitudinaldirection. When optical fiber is drawn from a completed base materialwith a large amount of outer diameter fluctuation, the clearance betweenthe base material insertion opening of the drawing device and the glassbase material for optical fiber 3 changes during the drawing, andtherefore the air flow within the furnace of the drawing device changes.As a result, the characteristics of the drawn optical fiber change inthe drawing direction, thereby lowering the quality of the opticalfiber.

Accordingly, by further decreasing the outer diameter fluctuation of thecompleted base material, the quality of the optical fiber can beimproved. Therefore, when processing the glass base material for opticalfiber 3 to form the completed base material, the amount of diameterreduction is increased at locations of the glass base material foroptical fiber 3 with a large outer diameter and the amount of diameterreduction is decreased at locations of the glass base material foroptical fiber 3 with a small outer diameter, thereby forming a completedbase material with a small amount of outer diameter fluctuation.

However, in a case where the average diameter reduction amount for theglass base material for optical fiber 3 is 5 mm or more, for example,the heating capacity of the glass base material for optical fiber 3fluctuates locally at locations in front of and behind locations wherethe outer diameter fluctuation is large. As a result, portions with asmall diameter experience more diameter reduction than portions with alarge diameter, which emphasizes the outer diameter fluctuation.Furthermore, the same phenomenon occurs when local outer diameterfluctuation in the glass base material for optical fiber 3 is shorterthan the length of the uniform heating band of the heat source 5.Therefore, it is difficult to manufacture a completed base material witha small amount of outer diameter fluctuation.

FIG. 2 shows the flow of the procedure performed when processing theglass base material for optical fiber 3 to form the completed basematerial. The following further describes the procedure shown in thedrawing.

Step 1:

First, at Step 1, the outer diameter of the glass base material foroptical fiber 3 that is to undergo the elongation process and become thecompleted base material is measured. The outer diameter measurement isan outer diameter distribution obtained by measuring the outer diameterof the glass base material for optical fiber 3 at a plurality ofmeasurement positions arranged at short measurement intervals in thelongitudinal direction of the glass base material for optical fiber 3.Based on this outer diameter distribution, the final elongationdiameter, which is the outer diameter of the glass base material foroptical fiber 3 that has finally become the completed base material, canbe set.

As a specific example, measurement of the outer diameter and movement ofthe glass base material for optical fiber 3 having the dummy rods 2attached thereto and held by the glass lathe 1 in the longitudinaldirection are repeated. As a result, the length of the glass basematerial for optical fiber 3 is also measured. The small measurementintervals are preferably as narrow as possible within a range allowableby the work time and other factors, and may be intervals that are nogreater than 10% of the width of the uniform heating band, which is theband across which the heating temperature of the heat source 5 isuniform, in the longitudinal direction of the glass base material foroptical fiber 3.

Step 2:

Next, at step 2, the effective region is designated for the glass basematerial for optical fiber 3 to undergo processing. The effective regionis designated as one effective region of one glass base material foroptical fiber 3 that is continuous in the longitudinal direction, inconsideration of optical uniformity, the presence of air bubbles, andthe like. The effective region is designated such that the volume of theglass base material for optical fiber 3 within this region is greaterthan or equal to the volume of the completed base material calculatedfrom the final elongation diameter and length at completion.

The volume of the effective region is calculated by using themeasurement data obtained via the outer diameter measurement describedabove and calculating the volume of a conical portion assumed to existbetween a measurement position and an adjacent measurement position,based on the outer diameter measurement values at each of thesemeasurement positions. The effective region may be set to include anallowance for the volume of the completed base material that isapproximately 10% of the volume of the glass base material for opticalfiber 3. In this way, it is possible to compensate for changes in volumecaused by the scattering of silica glass from the surface during theelongation processing.

The elongation process performed on the glass base material for opticalfiber 3 by the glass lathe 1 can decrease the outer diameter of theglass base material for optical fiber 3, but cannot increase the outerdiameter of the glass base material for optical fiber 3. Accordingly,when setting the effective region, there is a condition that the outerdiameter of the glass base material for optical fiber 3 must be greaterthan or equal to the final elongation diameter across the entire lengthof the effective region.

Step 3:

Next, at step 3, the average outer diameter and the outer diameterfluctuation value are calculated for the set effective region of theglass base material for optical fiber 3, and an investigation is made asto whether the outer diameter difference between the calculated averageouter diameter and the final elongation diameter described above exceedsa predetermined outer diameter difference reference value. If thisdifference does exceed the outer diameter difference reference value(Step 3: NO), then the process moves to step A where a target elongationdiameter that is greater than the final elongation diameter is set andthe glass base material for optical fiber 3 is elongated.

The elongation amount needed to reach the set target elongation diameteris less than or equal to the outer diameter difference reference valuedescribed above. Furthermore, the elongation of the glass base materialfor optical fiber 3 in step A is performed using a movement speed andheating amount for the heat source 5 determined on a condition that theelongation changes the outer diameter from the calculated average outerdiameter to the target elongation diameter.

After this, the outer diameter measurement (Step 1), the effectiveregion designation (Step 2), and the calculation of the differencebetween the average outer diameter and the final elongation diameter(Step 3) are repeated on the glass base material for optical fiber 3that has been elongated to realize the target elongation diameter. Steps1 to 3 described above are repeated until, at Step 3, the differencebetween the average outer diameter and the final elongation diameter isless than or equal to the outer diameter difference reference value(Step 3: YES).

If the outer diameter of the glass base material for optical fiber 3 ismeasured at uniform intervals in the longitudinal direction, the averageouter diameter calculated at Step 3 may be calculated as the simplearithmetic average of the measurement values of the outer diameter. Ifthe outer diameter measurement positions are not at uniform intervals onthe glass base material for optical fiber 3, the average outer diametercalculated at Step 3 may be calculated as a weighted average value thattakes into consideration the intervals between the measurementpositions.

The outer diameter difference reference value is preferably less than orequal to 10% of the final elongation diameter. For example, whenprocessing a glass base material for optical fiber 3 with an averageouter diameter of 80 mm to form a completed base material with a finalelongation diameter of 50 mm, the outer diameter difference referencevalue is preferably less than or equal to 5 mm.

The target elongation diameter set in Step A may be set such that thedifference between the average outer diameter and the final elongationdiameter resulting from one instance of the elongation process is lessthan or equal to the outer diameter difference reference value, or Steps1 to 3 may be repeated until the difference between the average outerdiameter and the target elongation diameter is less than or equal to theouter diameter difference reference value. In this case, the targetelongation diameter may be calculated using Expression 1 shown below.

Expression 1:

{average outer diameter×k+final elongation diameter×(1−k)}

In the above expression, k may be a value that is greater than or equalto 0.25 and less than or equal to 0.75, where a larger value for kindicates a greater number of elongation steps.

Using the example described above, after performing the first elongationstep of the elongation process in which the target elongation value is adiameter of 60 mm (k=0.33 for the average outer diameter of 80 mm andthe final elongation diameter of 50 mm), the second elongation step ofthe elongation process may be performed with a target elongation valueof a diameter of 53 mm (k=0.3 for the average outer diameter of 60 mm),for which the outer diameter difference between the average outerdiameter and the final elongation diameter is within 10% of the finalelongation diameter.

In Expression 1 above, if the value of k is less than 0.25, thedifference between the outer diameter of the glass base material foroptical fiber 3 before elongation and the outer diameter afterelongation is small, and therefore the effect of reducing the outerdiameter fluctuation is difficult to realize. Furthermore, since thenumber steps repeated in the elongation process increases, themanufacturability of the completed base material is worse. Furthermore,in Expression 1 above, if the value of k is greater than 0.75, when theelongation process is performed, the regions with localized smalldiameters experience further diameter reduction, which might cause theouter diameter to be less than the final elongation diameter.Accordingly, the value of k is preferably in a range that satisfiesExpression 2 below.

Expression 2:

0.25≦k0.75

Furthermore, when considering improvements to both manufacturability andquality, the value of k may be in a range that satisfies Expression 3shown below.

Expression 3:

0.4≦≦k0.6

In addition, in order to simplify the work performed during theelongation process, k may be set to a value that allows for easycalculation, such as a value of 0.5.

Step 4:

If the difference between the average outer diameter and the finalelongation diameter in the effective region is less than or equal to theouter diameter difference reference value described above in Step 3(Step 3: YES), then at Step 4, the outer diameter of the glass basematerial for optical fiber 3 in the effective region is again measuredat small intervals, and the outer diameter fluctuation and local outerdiameter fluctuation value are calculated.

The outer diameter measurement is performed by measuring the outerdiameter of the glass base material for optical fiber 3 at smallmeasurement intervals in the longitudinal direction of the glass basematerial for optical fiber 3, in the same manner as in Step 1.Specifically, measurement of the outer diameter and movement of theglass base material for optical fiber 3 having the dummy rods 2 attachedthereto and held by the glass lathe 1 in the longitudinal direction arerepeated. As a result, the length of the glass base material for opticalfiber 3 is also measured. The small measurement intervals are preferablyas narrow as possible, and may be intervals that are no greater than 10%of the width of the uniform heating band, which is the band across whichthe heating amount of the heat source 5 is uniform, in the longitudinaldirection of the glass base material for optical fiber 3.

The outer diameter fluctuation value calculated in Step 4 may becalculated as the difference between the maximum outer diameter and theminimum outer diameter of the glass base material for optical fiber 3 asmeasured in the effective region. The local outer diameter fluctuationvalue refers to the highest value among regional outer diameterfluctuation values obtained by dividing the effective region of theglass base material for optical fiber 3 into a plurality of evaluationregions and, for each of these evaluation regions, measuring theregional outer diameter fluctuation value.

The width of each evaluation region set when calculating the local outerdiameter fluctuation value may be set to be the width of the uniformheating band of the heat source 5. If a heating resistor is used as theheat source 5, the width of the uniform heating band of the heat source5 may be treated as being substantially equal to the width of theheating resistor itself in the longitudinal direction of the glass basematerial for optical fiber 3. If a flame burner is used as the heatsource 5, the width of the uniform heating band may be treated as beingapproximately three times the diameter of the burner opening.

Step 5:

Next, at Step 5, an investigation is made as to whether the outerdiameter fluctuation value within the effective region does not exceedthe outer diameter fluctuation reference value. If the outer diameterfluctuation value does exceed the predetermined outer diameterfluctuation reference value (Step 5: NO), Step A described above isperformed to set a target elongation diameter that is greater than thefinal elongation diameter, the elongation process is again performed onthe glass base material for optical fiber 3, and then the processreturns to Step 1 and Steps 1 to 4 are repeated. In this way,excessively large outer diameter fluctuation values are lowered.

The target elongation diameter set when the process moves to Step Aafter Step 5 may also be set such that the difference between theaverage outer diameter and the final elongation diameter resulting fromone instance of the elongation process is less than or equal to theouter diameter difference reference value, or Steps 1 to 4 may berepeated until the difference between the average outer diameter and thetarget elongation diameter becomes less than or equal to the outerdiameter difference reference value. In this case as well, the targetelongation diameter may be calculated using Expression 1 shown above.

The outer diameter fluctuation reference value described above may bewithin 1/25, i.e. 4%, of the final elongation diameter, for example. Inthe example described above, when the final elongation diameter is 50mm, for example, the outer diameter fluctuation reference value is setto a predetermined value of less than or equal to 2 mm. At Step 5, ifthe outer diameter fluctuation value within the effective region is lessthan or equal to the outer diameter fluctuation reference value (Step 5:YES), the process moves to Step 6.

Step 6:

At Step 6, an investigation is made as to whether the local outerdiameter fluctuation value of the glass base material for optical fiber3 does not exceed a predetermined local outer diameter fluctuationreference value. If the local outer diameter fluctuation value doesexceed the predetermined local outer diameter fluctuation referencevalue (Step 6: NO), Step A is performed to set a target elongationdiameter that is greater than the final elongation diameter, theelongation process is again performed on the glass base material foroptical fiber 3, and then the process again returns to Step 1 and Steps1 to 5 are repeated. In this way, excessively large outer diameterfluctuation values occurring locally are reduced.

If there is a large outer diameter fluctuation remaining in any of theevaluation regions used when calculating the local outer diameterfluctuation value, outer diameter fluctuation that has not beencorrected might remain after the elongation process is performed toreduce the diameter of the glass base material for optical fiber 3.Accordingly, the local outer diameter fluctuation reference value ispreferably within ⅔ of a final outer diameter fluctuation allowablevalue, which is an allowable value for the outer diameter fluctuationvalue in the completed base material. In the example described above,when the final outer diameter fluctuation allowable value is 5% of thefinal elongation diameter, the final outer diameter fluctuationallowable value is 2.5 mm for a completed base material with a finalelongation outer diameter of 50 mm. Accordingly, the local outerdiameter fluctuation reference value may be set to 1.7 mm, whichcorresponds to ⅔ of the final outer diameter fluctuation allowablevalue.

The target elongation diameter set when the process moves to Step Aafter Step 6 may also be set such that the difference between theaverage outer diameter and the final elongation diameter resulting fromone instance of the elongation process is less than or equal to theouter diameter difference reference value, or Steps 1 to 4 may berepeated until the difference between the average outer diameter and thetarget elongation diameter becomes less than or equal to the outerdiameter difference reference value. In this case as well, the targetelongation diameter may be calculated using Expression 1 shown above. AtStep 6, if the local outer diameter fluctuation value within theeffective region is less than or equal to the local outer diameterfluctuation reference value (Step 6: YES), the process moves to Step 7.

Step 7:

At Step 7, the glass base material for optical fiber 3 is processeduntil reaching the final elongation diameter, thereby manufacturing thecompleted base material. In this way, respective steps are performed toset the difference between the final outer diameter and the averageouter diameter to be less than or equal to the outer diameter differencereference value, to set the outer diameter fluctuation to be less thanor equal to the outer diameter fluctuation reference value, and to setthe local outer diameter fluctuation value to be less than or equal tothe local outer diameter fluctuation reference value, and after thesesteps the elongation process is performed to manufacture the completedbase material, thereby realizing manufacturing of a completed basematerial having high outer diameter precision, with high yield andefficiency. Furthermore, by measuring the outer diameter of the glassbase material for optical fiber 3 at small intervals in the longitudinaldirection before the elongation, the effective region can be set withprecision and it is possible to ensure that the length of the completedbase material is greater than the target length.

The series of steps described above in the elongation process using theglass lathe 1 to form the completed base material from the glass basematerial for optical fiber 3 can be automated. In other words, beforethe elongation process, the outer diameter of the glass base materialfor optical fiber 3 may be measured at small intervals in thelongitudinal direction and the elongation speed may be adjusted torealize an elongation amount, i.e. a diameter reduction amount,corresponding to the outer diameter at the heating positions. Here,after setting the heating amount by the heat source 5 to be sufficientfor softening the glass base material for optical fiber 3, while heatingthe glass base material by moving the heat source 5 relative to theglass base material in one direction at a constant speed in thelongitudinal direction of the glass base material, the movement speedsof the chucks are adjusted such that the space between the left andright chucks increases according to the outer diameter of the portionbeing heated.

The adjustment amount of the adjustment described above is strictlycalculated by taking the mass balance of the length of the widened spacebetween the chucks and the target elongation diameter, relative to themovement distance of the heat source and the local outer diameter priorto processing. Specifically, in a given reaction system, the generalrelationship shown below is established. (inflow amount to thesystem)=(outflow amount from the system)+(accumulation amount within thesystem)

When processing the glass base material for optical fiber 3 as describedabove, the accumulation amount within the system can be treated as beingzero, and therefore the mass balance can be realized by calculating themovement amount (movement speed) of the chucks (and the heat source),such that the relationship shown below is established. (inflow amount atthe heating position)=(outflow amount from the heating position)

Accordingly, using the glass lathe 1 shown in FIG. 1, on the conditionthat the heat source 5 is fixed, in a case where a base material with anouter diameter of D₁ is input at a speed of V₁ from the left side in thedrawing and the elongated base material with a diameter of D₂ and isoutput at a speed of V₂ from the right side, the mass balance can berealized based on Expression 4 shown below.

Expression 4:

πρD ₁ ² V ₁ =πρD ₂2V ₂

Here, π is the circumference ratio and ρ is the density.

Furthermore, the above relationship is also established in a case where,in the glass lathe 1, the chuck on the left side in the drawing isfixed, the heat source 5 is moved to the left of the drawing at a speedof −V₁, and the chuck on the right side in the drawing is moved at aspeed of V₂−V₁.

In this way, the in the method of processing the glass base material foroptical fiber 3 described above, when the outer diameter differencebetween the average outer diameter and the final elongation diameter,the outer diameter fluctuation value, and the local outer diameterfluctuation value exceed respective predetermined reference values, atarget elongation diameter larger than the final elongation diameter isdetermined and the elongation steps are repeated until the this targetelongation diameter is reached and the outer diameter difference, theouter diameter fluctuation value, and the local outer diameterfluctuation are less than or equal to the respective reference values,at which point the elongation process is performed until reaching thefinal elongation diameter. In this way, it is possible to efficientlyand precisely set the outer diameter of the glass base material.Furthermore, it is possible to ensure that the completed base materialhas a length greater than or equal to the target length. The elongationsteps performed during the processing can proceed efficiently accordingto comparisons with each type of reference value, and therefore there isno need for additional unnecessary elongation steps, so that theprocessing time can be shortened.

In the example described above, the target elongation diameter that islarger than the final elongation diameter is calculated based on thereference values for the average outer diameter, the outer diameterfluctuation value, and the local outer diameter fluctuation value in theeffective region of the glass base material for optical fiber 3, andthen the process for elongating the glass base material for opticalfiber 3 is performed. However, among the average outer diameter, theouter diameter fluctuation value, and the local outer diameterfluctuation value, the reference values of one or both of the outerdiameter fluctuation value and the local outer diameter fluctuationvalue may be omitted from the determination, and Step 7 of theelongation process may be performed until reaching the final elongationdiameter. In this case, some or all of Steps 4 and 5 shown in FIG. 2 areomitted.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated by “prior to,” “before,” or the like and as longas the output from a previous process is not used in a later process.Even if the process flow is described using phrases such as “first” or“next” in the claims, embodiments, or diagrams, it does not necessarilymean that the process must be performed in this order.

LIST OF REFERENCE NUMERALS

1: glass lathe, 2: dummy rod, 3: glass base material for optical fiber,4: diameter measurement device, 5: heat source

What is claimed is:
 1. A method of processing a glass base material foroptical fiber in which the glass base material for optical fiber iselongated to reduce a diameter thereof until reaching a final elongationdiameter and form a completed base material, the method comprising:before elongating the glass base material for optical fiber, measuringan outer diameter distribution that includes an outer diameter of theglass base material for optical fiber at a plurality of measurementpoints in a longitudinal direction of the glass base material foroptical fiber; setting an effective region that is continuous in thelongitudinal direction in the glass base material for optical fiber,based on the measured outer diameter; calculating a target elongationdiameter that is larger than the final elongation diameter and less thanan average diameter of the effective region of the glass base materialfor optical fiber that is calculated based on the measured outerdiameter, and elongating the glass base material for optical fiber untilreaching the target elongation diameter; and after reaching the targetelongation diameter, further elongating the glass base material foroptical fiber until reaching the final elongation diameter.
 2. Themethod of processing the glass base material for optical fiber accordingto claim 1, comprising: before elongating the glass base material foroptical fiber, calculating an outer diameter fluctuation value that is adifference between a maximum outer diameter and a minimum outer diameterof the glass base material for optical fiber in the effective region, inthe longitudinal direction of the glass base material for optical fiber;calculating the target elongation diameter based on the average outerdiameter of the glass base material for optical fiber in the effectiveregion and the outer diameter fluctuation value of the glass basematerial for optical fiber in the effective region, and elongating theglass base material for optical fiber until reaching the targetelongation diameter; and after reaching the target elongation diameter,further elongating the glass base material for optical fiber untilreaching the final elongation diameter.
 3. The method of processing theglass base material for optical fiber according to claim 1, wherein thetarget elongation diameter is calculated according to Expression 1,Expression 1 is {average outer diameter×k+final elongationdiameter×(1−k)}, k satisfies Expression 2, andExpression 2 is 0.25≦k≦0.75.
 4. The method of processing the glass basematerial for optical fiber according to claim 3, wherein a value of ksatisfies Expression 3, andExpression 3 is 0.4≦k≦0.6.
 5. The method of processing the glass basematerial for optical fiber according to claim 1, wherein afterelongating the glass base material for optical fiber until reaching thetarget elongation diameter and before elongating the glass base materialfor optical fiber until reaching the final elongation diameter, theouter diameter of the glass base material for optical fiber is measuredat a plurality of measurement points in the longitudinal direction ofthe glass base material for optical fiber and the average outer diameterof the glass base material for optical fiber in the effective region iscalculated, if an outer diameter difference that is a difference betweenthe average diameter and the final elongation diameter is greater than apredetermined outer diameter difference reference value, the glass basematerial for optical fiber is elongated until reaching the targetelongation diameter that is less than the average outer diameter andmore than the final elongation diameter, and if the outer diameterdifference is less than or equal to the outer diameter differencereference value, the glass base material for optical fiber is elongateduntil reaching the final elongation diameter.
 6. The method ofprocessing the glass base material for optical fiber according to claim5, wherein the outer diameter difference reference value is less than orequal to 10% of the final elongation diameter.
 7. The method ofprocessing the glass base material for optical fiber according to claim1, wherein after elongating the glass base material for optical fiberuntil reaching the target elongation diameter and before elongating theglass base material for optical fiber until reaching the finalelongation diameter, the outer diameter of the glass base material foroptical fiber is measured at a plurality of measurement points in thelongitudinal direction of the glass base material for optical fiber andan outer diameter fluctuation value that is a difference between amaximum diameter and a minimum diameter of the glass base material foroptical fiber in the effective region is calculated, if the outerdiameter fluctuation value is greater than a predetermined outerdiameter fluctuation reference value, the glass base material foroptical fiber is elongated until reaching the target elongation diameterthat is less than the average outer diameter and more than the finalelongation diameter, and if the outer diameter fluctuation value is lessthan or equal to the outer diameter fluctuation reference value, theglass base material for optical fiber is elongated until reaching thefinal elongation diameter.
 8. The method of processing the glass basematerial for optical fiber according to claim 7, wherein the outerdiameter fluctuation reference value is less than or equal to 1/25 ofthe final elongation diameter.
 9. The method of processing the glassbase material for optical fiber according to claim 1, wherein afterelongating the glass base material for optical fiber until reaching thetarget elongation diameter and before elongating the glass base materialfor optical fiber until reaching the final elongation diameter, theeffective region is divided into a plurality of measurement regions inthe longitudinal direction of the glass base material for optical fiberand a local outer diameter fluctuation value that is a differencebetween a maximum diameter and a minimum diameter of the glass basematerial for optical fiber in each of the measurement regions iscalculated, if the local outer diameter fluctuation value is greaterthan a predetermined local outer diameter fluctuation reference value,the glass base material for optical fiber is elongated until reachingthe target elongation diameter that is less than the average outerdiameter of the glass base material for optical fiber in the effectiveregion and more than the final elongation diameter, and if the localouter diameter fluctuation value is less than or equal to the localouter diameter fluctuation reference value, the glass base material foroptical fiber is elongated until reaching the final elongation diameter.10. The method of processing the glass base material for optical fiberaccording to claim 9, wherein each of the plurality of measurementregions has a width corresponding to a width of a uniform heating bandof a heat source that performs heating when elongating the glass basematerial for optical fiber.
 11. The method of processing the glass basematerial for optical fiber according to claim 9, wherein the local outerdiameter fluctuation value is less than or equal to ⅔ of an allowablevalue for the outer diameter fluctuation in the completed base material.12. The method of processing the glass base material for optical fiberaccording to claim 1, wherein the effective region of the glass basematerial for optical fiber has a volume that is greater than or equal toa volume of the completed base material calculated from the finalelongation diameter and the length of the completed base material.